The present invention relates generally to a load current analysis system for use with an electrical utility power system, and more particularly to a load extraction fault detection system for detecting high impedance, low current arcing faults on the power system. Arcing faults may be caused by, for example, downed, broken, tangled or dangling power lines, trees contacting the power lines, and various overcurrent fault situations.
Arcing faults are more difficult to detect than permanent overcurrent faults, which for instance, occur when a transformer fails. Most conventional overcurrent protection devices, such as fuses, reclosures, relays and the like, have time delays which prevent a temporary fault from de-energizing the power line. Only if the overcurrent fault persists does such a protection device de-energize the power line. Some arcing faults may initialize the timing circuits of the overcurrent protection devices but, by the end of the time delay, the high impedance nature of the fault limits the fault current to a low value. Such overcurrent protection devices cannot distinguish this low fault current from the levels of current ordinarily drawn by customers; hence, the line may remain energized even though a dangerous arcing fault exists on the power line.
Other methods of detecting arcing faults have focused on comparing voltage and current signals. For instance, U.S. Pat. No. 4,871,971 to Jeerings et al. ("Jeerings") detects an abnormality on an electric power system based upon the phasor relationship between the fundamental voltage and a harmonic current, specifically, the third harmonic. Jeerings detects a high impedance fault based upon a predetermined change in this phasor relationship. Thus, Jeerings lacks any dynamic capability to track the actual line current.
To implement Jeerings' method requires expensive filtering to produce the third harmonic component. Moreover, Jeerings uses phasor comparisons which are calculated over relatively long periods of time, yielding a slow response to a potentially dangerous situation. For example, Jeerings compares the phasor relationships determined over a short term (up to 5 seconds) with those derived over a relatively long term (10 seconds to 10 minutes).
Another deficiency of Jeerings is that the harmonic current is usually not related to the voltage or current levels of the power line during a high impedance fault. Instead, the current drawn by an arcing fault is often dependent on the environmental conditions at the fault site, such as conductor movement, the presence of ionized gases and/or soil in the current path, and the type of grounding surface in contact with the live conductor. Clearly Jeerings lacks any adaptability to accommodate such rapidly changing environmental situations at a fault site.
Another extraction technique for detecting high impedance faults was proposed by J. Carr and G. L. Hood in an article entitled, "High Impedance Fault Detection on Primary Distribution Systems," Canadian Electrical Association Final Report, Project No. 78-75, 1979. Carr and Hood eliminate load current from the total current during a fault condition using analog circuitry known as a "wash-out" filter. Basically, Carr and Hood use a frequency filtering technique, i.e., a high pass filter, which can inherently lead to inaccuracies. The Carr and Hood technique deals only with RMS current values, rather than with waveforms.
Thus, a need exists for an improved load extraction arcing fault detection system for electrical power utilities which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.